3.3.1. Dry Friction against Steel

The dry friction value of (AlCr)N coatings against steel is slightly higher than that of CrN at ca. 0.65 [19], as measured using the pin–on-disc method against hardened bearing steel, 100Cr6. A friction value for (AlCr)N of about 0.5 was reported for AISI4340 as a pin [82]. The friction value at low load (5 N) was similar to that of (TiAl)N, whereas at high load conditions (20 N), (AlCr)N displayed a significantly lower friction coefficient. The superior tribological behaviour, namely a low wear rate and low friction, of an (AlCr)N coating against AISI 4340 steel is associated with the higher proportion of distinct (dispersive-polar) interactions, as evidenced by the measured surface energies. (AlCr)N and (TiAl)N coatings have a high polar component of 19.8 and 15.4 mJ/m2, and a low dispersive component of 2.9 and 6.5 mJ/m2, respectively, while the AISI 4340 steel presented a similar proportion between the polar, 14.3 mJ/m2 and dispersive 13.5 mJ/m2 components [82]. Measurement of the dry friction of (AlCr)N coatings by pin-on-disc against austenitic stainless steel (DIN1.4301, AISI 304, hardness 274 HV1) showed a friction value of 0.85 at room temperature, which dropped to about 0.6 at higher temperatures [83].

## 3.3.2. Dry Friction and Wear against Ceramics

The coefficient of friction measured by pin-on-disc tests against Al2O3 balls for (CrAl)N and (AlCr)N coatings of different Al contents showed a friction coefficient of about 0.6 at room temperature, independent of the Al content [83]. The friction value increased for 500 ◦C and dropped to 0.6 again at 700 ◦C, and was also nearly independent of the Al content. Figure 15 shows the decreasing abrasive wear rate with increasing Al content for all investigated temperatures [83].

**Figure 15.** Wear coefficient of (CrAl)N and (AlCr)N coatings with different Al contents by pin-on-disc tests compared to Al2O3 balls for different test temperatures, redrawn after [83], original © Springer Nature.

The dry tribological properties of arc-deposited (Al70Cr30)N, (Al67Ti33)N, and CrN coatings against a Si3N4 ball as the counterpart showed friction values of 0.73, 0.79, and 0.77 at the end of the test, respectively [84]. A special effect related to the wear rate was observed. The highest wear rate was measured for (Al67Ti33)N, but CrN had a slightly lower wear rate than (Al70Cr30)N. The authors claim this is caused by the tribological oxidation behaviour, which has a great influence on the wear mechanism and the debris removal behaviour of the coatings.

#### *3.4. Control of Coating Morphology, Stress and Texture*

(CrAl)N and (AlCr)N coatings synthesised using PVD processes display a rich variety of microstructures, from fine-grained morphology up to coarse columnar structures, which make different (AlCr)N coatings suitable for many diverse application areas. This section focuses on (AlCr)N coatings, though similar effects are also valid for (CrAl)N coatings. The relationships between the deposition parameters and the coating morphology are summarised in generic structure-zone diagrams [85] to provide guidelines that can be applied for (AlCr)N coatings in general. The deposition conditions can also be used to control the stress state of the coatings [86], which often is an important part of engineering coatings and in adapting them to different use cases and requirements. For example, thick coating layers are at risk of peeling off if the stress state is too high, while high compressive stresses can be advantageous in interrupted cutting operations. The stress state has also been pointed out as a key determining factor for thermal stability and decomposition pathways [75].

The effect of bias voltage on the stress state and coating properties of (AlCr)N coatings deposited using Al60Cr40 targets has been investigated systematically [87]. Increasing the bias voltage led to higher levels of compressive residual stresses as a function of the increased ion bombardment, up to a threshold in the range of 100 V, see Figure 16. Further higher bias values reduced the overall stress state, which was attributed to the annihilation of defects and stress relaxation. A concurrent reduction in grain size, the lattice parameter and a modified preferential orientation in XRD from (200) to a mixture of (200) and (111) were also observed as an effect of increasing the bias [87].

**Figure 16.** Compressive residual stress versus the bias voltage for arc-deposited (Al60Cr40)N coatings, redrawn after [87], original © Elsevier.

Similar trends for texture and increased hardness with bias were also reported for coatings deposited using Al70Cr30 and Al75Cr25 cathodes. Coatings with a higher Al content, deposited using Al85Cr15 and Al90Cr10, formed dual-phase structures at bias voltages higher than 40 V [66]. The influence of phase structure on the stress state has been exemplified in the direct comparison of (Al80Cr20)N coatings, which had lower compressive stress values relative to (Al70Cr30)N. This effect was attributed to stress relief through the presence of hexagonal phases in the former case [88].

The stress state of (AlCr)N coatings can also be influenced by alloying elements. For example, B-alloyed coatings with 2.3–9.1 at.% of B have been demonstrated to have lower compressive state levels compared to unalloyed coatings. The B-alloying also caused grainsize refinement and an increase in hardness attributed to a combination of solid solution hardening and Hall-Petch hardening [89]. Further aspects of alloying will be discussed later in this review.

Surface morphology, and in particular the density of macroparticles in arc deposition processes, can be influenced by the deposition pressure where fewer macroparticles are

generated and smoother coatings are obtained at higher pressure [90–92], see also the example in Figure 17. This effect can be rationalised through a higher degree of poisoning on the target surface at an elevated pressure. Columnar morphology is typically achieved over a large process window of deposition pressures, though with variation in the column width. This is illustrated in Figure 17a–d. Further reported effects of deposition pressure include increased hardness and an influence on the phase structure [91,92].

**Figure 17.** Surface and cross-sectional morphology of (AlCr)N monolayer coatings deposited for (**a**,**c**) at 2.5 Pa N2 and for (**b**,**d**) at 7.5 Pa N2, for illustration of relative trends.

The structure of arc-deposited AlCr-based coatings depends not only on the cathode composition, process temperature, substrate bias potential, arc current and reactive gas pressure, but also on the arc source design, in a manner that is similar to the case of AlTi-based coatings [9]. The influence of the magnetic set-up was investigated in one coating process using two arc sources, both equipped with Al70Cr30 cathodes, strong and weak magnetic fields, respectively [93]. Figure 18 shows the differences between the two deposited coatings by means of XRD investigation, SEM cross-section and hardness measurement. The coating properties vary significantly. Although EPMA (Electron Probe Micro Analysis) measurements showed a lower Al/(Al + Cr) content, a minor hcp face is visible in the coating deposited by the weak field, which explains the finer growth structure (smaller grains) of the coating. Furthermore, the coating deposited with the weak field contains less nitrogen and the hardness is about 30% lower compared to the coating deposited with strong magnetic fields.

### *3.5. Features of (AlCr)N Coatings with Mixed fcc Plus hcp Structure*

Dual-phase coatings with Al concentrations of more than 70 at.% have been reported as monolayer coatings [88,90,94,95], as well as multilayers [66,96]. The coatings are characterised by relatively low hardness compared with fcc (Al,Cr)N, lower oxidation resistance than fcc (Al,Cr)N with a high Al content, but also with the positive effect of exhibiting lower stress.

**Figure 18.** Phase formation of coatings deposited with pm cathodes Al70Cr30 for two evaporators with different magnetic field set-ups [93].
